New Standard for Evaluating Casing Connections for Thermal-Well Applications

Abstract

Summary Casing connections in thermal wells experience extreme loads owing to exposure to high temperatures ranging as high as 200 to 350°C, stresses exceeding the elastic limit, and cyclic plastic deformation. To date, no standard procedure has been adopted by the industry to qualify casing connections for such conditions. In particular, the existing evaluation standard published jointly by the International Organization for Standardization (ISO) and American Petroleum Institute (API) ISO 13679/API 5C5 (ISO 2002) does not apply to tubular loads beyond pipe body yield and temperatures above 180°C. Proprietary procedures have been used to qualify connections for individual thermal operations, but none of those has been accepted as an industry standard. This paper introduces a new protocol for evaluating casing connections for thermal applications: Thermal Well Casing Connection Evaluation Protocol (TWCCEP). While founded on longstanding work in the thermal-well arena, TWCCEP was developed through a multiclient project sponsored by several major operators and connection manufacturers. Following project completion, TWCCEP was released to the public domain and submitted to ISO for adoption as an international standard. TWCCEP uses both analytical and experimental procedures to assess performance of a candidate connection under conditions typical of service in thermally stimulated wells. The objective of the analytical component is to assess sensitivities of the candidate connection to selected design variables, and identify worst-case combinations of those variables for configuration of specimens for physical testing. The purpose of the physical testing is to verify performance of the connection specimens under assembly-and- loading conditions simulating thermal-well service. In addition to an overview of TWCCEP, this paper illustrates how engineering analysis, numerical simulation, and reduced-scale physical testing were used to examine the impacts of various design and loading variables on connection strength and sealability, and subsequently to formulate the analysis-and-test matrix prescribed in the TWCCEP evaluation procedure. Adoption and consistent use of TWCCEP is expected to increase operational reliability and decrease failure potential of casing strings in thermal wells. Learnings from the protocol development process will also help in the formulation of guidelines for connection requalification for thermal applications, in cases when one or more of the design variables change.